Field of the Invention
Methods and apparatuses consistent with the present invention relate to communication in a wireless network, and more particularly, to communication using a direct link in a wireless network and an apparatus therefor.
Description of the Related Art
A method of using direct link protocol (DLP) and multi-channels in a basic service set (BSS) in order to maximize transmission efficiency in a wireless network has been suggested. Unlike a wired network, in the wireless network, communication may be interrupted by an external factor such as channel interference or noise. When a transmission interrupt occurs in a wireless communication system using a plurality of channels in the BSS, it is important to perform a fast handover to another available channel.
FIG. 1 illustrates an example of a roaming method performed by a station in a related art wireless communication environment.
FIG. 1 illustrates a procedure in which a mobile station 112 roams to a subnet Y 120 when the mobile station 112 in a subnet Z 130 is connected to another mobile station in a subnet X 110 through a quality of service (QoS) application. A related art operation performed in a media access control (MAC) layer when the mobile station 112 roams from the subnet X 110 to the subnet Y 120 in a wireless (local area network) LAN, will now be described. Here, it is assumed that a practical roaming operation is performed in an extended service set (ESS), and the subnet X 110 and the subnet Y 120 are included in the same ESS and overlap with each other.
When the roaming mobile station moves toward the subnet Y 120, and signal strength decreases, the roaming mobile station starts to search for a new channel for roaming. When searching for the channel in a passive mode, the roaming mobile station receives a beacon frame from the subnet Y 120. When searching for the channel in an active mode, the roaming station sends a probe request. The mobile station selects an access point (AP) that sends a stronger beacon frame or probe response than other APs.
In this example, the mobile station selects a new AP from an AP of the subnet Y 120 and sends a reassociation request to the new AP. The reassociation request includes information on the previous AP (the AP of the subnet X) and the mobile station itself. The previous AP does not recognize when the mobile station performs a handoff. The new AP informs the previous AP that the mobile station performs a handoff through an inter-access point protocol (IAPP). The mobile station joins a new channel through roaming by using the AP and starts sending and receiving data.
In the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, communication methods that use an AP include a communication method using a distributed coordination function (DCF) and a communication method using a point coordination function (PCF) and the DCF.
FIG. 2 illustrates a procedure of transmitting data between stations according to a DCF rule. A transmitting station (STA1) 210 checks whether a receiving station (STA2) 220 in the same BSS receives data by sending an request to send (RTS) frame 212 before transmitting data.
The STA2 220 sends a clear to send (CTS) frame 222, which is a control frame indicating that the STA2 can receive the frame, and allows the STA1 to transmit data. Then, the STA1 transmits data. Network allocation vector (NAV) periods 232 and 234 can be set so that stations 230 other than the STA1 210 and STA2 220 in the same BSS know that the channel is busy and do not transmit data.
IEEE 802.11e has been introduced for complementing wireless LAN standards such as IEEE 802.11 in which QoS is weak. In IEEE 802.11e, in order to improve QoS, the AP manages channel duration and the transmission order of the nodes.
IEEE 802.11e supports multiple frame transmission; IEEE 802.11 only supports single frame transmission. Although network throughput is improved through multiple frame transmission, in an infrastructure mode, frame transmission has to be through the AP, and therefore, there is a problem in efficiency of performance of the network. In order to solve this problem, DLP is suggested so that the nodes directly communicate with each other without intervention of the AP, thereby improving the performance of the network. In DLP defined in the IEEE 802.11e, when the infrastructure mode is used in BSS, data can be communicated using an independent link between stations without intervention of the AP while communicating data. In addition, the DLP stably manages the channel using the AP and provides maximum throughput by direct communication between stations. Since the DLP data is not transmitted through the AP during data communication, the transmission efficiency is increased by reducing transmission time, propagation time, and AP MAC processing time.
In order to communicate data using the DLP, the DLP has to be firstly set up. Hereinafter, the DLP setup process will be described in detail.
FIG. 3 illustrates a DLP set up process according to an exemplary embodiment of the present invention.
First, QSTA1 310, which is a DLP request station (DLP requester), sends a DLP request frame to an AP 320 (1a).
The DLP request frame includes a transmission data rate and information on the capability of a station. Next, the AP simply retransmits the DLP request frame to a QSTA2 330, which is a receiving station (1b). The QSTA2 330 confirms the DLP request that is received from the AP 320 and sends to the AP 320 (2a) a DLP response frame including information on whether the QSTA2 330 will join a direct link 340. The DLP response frame includes a state code for giving a result on the DLP request, the transmission data rate, and information on the capability of a station. Finally, the AP 320 simply retransmits the DLP response frame to the QSTA1 310 (2b). The aforementioned four processes are referred to as a 4-way handshake process.
FIG. 4 illustrates a DLP MAC frame according to a related art technique.
Referring to FIG. 4, actions are specified in a frame control field of a DLP MAC frame 400. The frame control field is a sub-field. A frame body field is divided into a category 410, a DLP action code 420, and DLP action bodies 430, 440, and 450.
A code ‘2’ which represents DLP is recorded in the category 410. Codes are specified in the DLP action code 420 according to types of the DLP MAC frame 400. The types of the DLP MAC frame 400 include a DLP request frame, a DLP response frame, and a DLP state frame.
The DLP action body 430 of the DLP request frame includes a destination MAC address field for recording a MAC address of a station that receives a frame, a source MAC address field for recording a MAC address of a station that transmits a frame, a capability field for recording functions which can be supported by the transmitting station, a transmission rate field for recording a transmission rate which is supported by the transmitting station, and an extended capability field.
The DLP action body 440 of the DLP response frame includes a state code field for recording whether a station accepts a DLP communication request, a MAC address field for a MAC address of the station that receives a current frame, a source address field for recording a MAC address of the station that transmits the current frame, a capability field for recording functions which can be supported by the transmitting station, a transmission rate field for recording a transmission rate which is supported by the transmitting station, and an extended capability field.
The DLP action body 450 of the DLP state frame includes a destination MAC address field, a source MAC address field, and a random data field.
In order to overcome a limit of transmission efficiency due to sharing a channel, the DLP has been suggested in 802.11e (QoS). When the DLP is used, data can be directly transmitted without passing through the AP, and therefore, a propagation delay and transmission times can be reduced. In addition, since a MAC processing time is not used in the AP, more data can be transmitted in a given time. However, since a limit of a current DCF in which one channel is shared is not overcome, as the number of stations increases, contention in a DCF segment increases, and therefore, the transmission efficiency decreases. In order to overcome the disadvantages due to sharing one channel, a method of using multiple channels using the DLP is suggested. However, in wireless communication, it is difficult to avoid channel interference and signal strength change due to the impact of the external environment on a physical layer.
When the multiple channels are employed by using the DLP and problems occur in an independent channel which the DLP stations use in the BSS due to internal or external factors, a method of changing the channel is needed. When the quality of the channel through which communication between DLP stations is performed deteriorates, it is necessary to maintain an effective and stable communication state by actively changing the channel.
In the communication using a wireless LAN, interference with radar signals may occur at 5 GHz. Accordingly, in European Telecommunications Standards Institute (ETSI) and Federal Communications Commission (FCC) in addition to International Telecommunications Union-Radiocommunications (ITU-R), a radar signal is detected, and communication is performed again through a new channel by avoiding a channel in which the radar signal is located. However, in the existing techniques, there is no method of avoiding a channel in which the radar signal is located.